Early computer data storage systems included one host device, one storage device (such as a hard disk drive, tape drive or an optical drive) and a storage controller to manage the transfer of data between the host and the storage device. The use of a single storage device eventually evolved into consolidating several storage devices (such as tape drives) into an automated library and many (sometimes hundreds or even thousands) of pieces of media (such as tape cartridges) retrievable by mechanical means for loading into one of the storage devices. More recent innovations permit more than one storage library to be attached to a host (or to more than one host) through a storage controller and multiple storage servers in such a fashion so as to be transparent to the host. Thus, regardless of the physical configuration of a storage system, the host logically “sees” only a single storage device through a single storage server. The storage servers in such a system are referred to as “virtual” storage servers (virtual tape servers in the case of tape-based systems) and the storage controllers are referred to as “virtual” storage controllers (virtual tape controllers in the case of tape-based systems).
Even more recently, IBM Corporation has developed a “Peer-to-Peer Virtual Tape Server”. Two (or possibly more) virtual tape server (VTSs) may be geographically separated (although they are not required to be) and are connectable to an S/390® host processor through a virtual tape controller (“VTC”), such as an IBM® Model AXO. VTC to VTS connections may be over enterprise system connection (ESCON®) or fiber connection (FICON TM) communication links, with or without channel extenders.
In a typical operation, a host sends a request to the controller for access to particular stored data. If the data resides in a server cache, the read or write operation is performed using the cached data. Otherwise, the data is retrieved from the storage device by the server and then the operation is performed.
Performance may degrade as the distance between the VTC and a VTS (and its corresponding library) increases. Therefore, when the distances from the VTC to one VTS is significantly greater than the distance to another VTS, it may be desirable to establish a preference for the closer VTS and library in an attempt to maximize performance. Data operations will be sent to the preferred VTS by default, absent the existence of other considerations, such as a backlog in the preferred server.
Each link between the VTC and a VTS may be characterized by the type of communications link connecting the two, the presence and type of any extender and the length of the link. The performance of a link is affected by each of these factors. For example, an ESCON link which is five kilometers (5 km) long will be faster than an ESCON link which is ten kilometers (10 km) long, other factors being equal. However, if the requested data resides in cache in the VTS attached through the longer link, the actual operation time may be faster through the VTS which is farther away from the VTC than if the data must be retrieved into the other, closer link.
Finally, during normal operations a closer VTS may have accumulated a backlog of host job requests and consequently that VTS may take longer to fulfill new requests than one which is farther from the VTC but has less of a backlog, regardless of the state of the cache.
Thus, there are significant interrelated variables which affect the overall performance of the storage system and there remains a need for improved workload allocation among storage servers in a storage system.